Many beneficial devices or structures in myriad applications are characterized at least in part by having a liquid or other fluid that is in contact with at least one solid surface or substrate. Many of these devices and applications are characterized by a liquid moving while in contact with a surface. Since the characteristics of both the liquid and the surface determine the interaction between the liquid and surface, it is often desirable to understand and control those characteristics to achieve control of the interaction of the liquid with those surfaces. For example, it is often desirable to control the flow resistance experienced by the liquid when it is in contact with the surface. Surfaces on which liquids or fluids exhibit a very low flow resistance are referred to herein as superhydrophobic surfaces.
FIGS. 1A-1E show different illustrative superhydrophobic surfaces produced using various methods. Specifically, these figures show substrates having small posts, known as nanoposts and/or microposts with various diameters and with different degrees of regularity. An illustrative method of producing nanoposts and microposts, found in U.S. Pat. No. 6,185,961, titled “Nanopost arrays and process for making same,” issued Feb. 13, 2001 to Tonucci, et al, is hereby incorporated by reference herein in its entirety. Nanoposts have been manufactured by various methods, such as by using a template, by various means of lithography, and by various methods of etching.
When a droplet of liquid, such as water, is placed on a substrate having an appropriately designed nanostructured or microstructured feature pattern, the flow resistance experienced by the droplet is dramatically reduced as compared to a droplet on a substrate having no such nanostructures or microstructures. Surfaces having such appropriately designed feature patterns are the subject of the article titled “Nanostructured Surfaces for Dramatic Reduction of Flow Resistance in Droplet-based Microfluidics”, J. Kim and C. J. Kim, IEEE Conf. MEMS, Las Vegas, Nev., January 2002, pp. 479-482, which is hereby incorporated by reference herein in its entirety. That reference generally describes how, by using surfaces with predetermined nanostructure features, the flow resistance to the liquid in contact with the surface can be greatly reduced. Specifically, the Kim reference teaches that, by finely patterning the surface in contact with the liquid, and using the aforementioned principle of liquid surface tension, a droplet of liquid disposed on the surface will be supported on the tops of the nanostructure pattern, as shown in FIG. 2. Referring to FIG. 2, droplet 201 of an appropriate liquid (depending upon the surface structure) will enable the droplet 201 to be suspended on the tops of the nanoposts 203 with no contact between the droplet and the underlying solid surface. This results in an extremely low area of contact between the droplet and the surface 202 (i.e., the droplet only is in contact with the top of each post 203) and, hence a low flow resistance.
In many applications, it is desirable to be able to control the penetration of a given liquid, such as the droplet 201 of FIG. 2, into a given nanostructured or microstructured surface, such as surface 202 in FIG. 2 and, thus, control the flow resistance exerted on that liquid as well as the wetting properties of the solid surface. FIGS. 3A and 3B show one embodiment in accordance with the principles of the present invention where electrowetting is used to control the penetration of a liquid into a nanostructured surface. Such electrowetting is the subject of copending U.S. patent application Ser. No. 10/403,159, filed Mar. 31, 2003, and titled “Method and Apparatus for Controlling the Movement of a Liquid on a Nanostructured or Microstructured Surface,” which is hereby incorporated by reference herein in its entirety. Referring to FIG. 3A, a droplet 301 of conducting liquid is disposed on a nanostructure feature pattern of conical nanoposts 302, as described above, such that the surface tension of the droplet 301 results in the droplet being suspended on the upper portion of the nanoposts 302. In this arrangement, the droplet only covers surface area f, of each nanopost. The nanoposts 302 are supported by the surface of a conducting substrate 303. Droplet 301 is illustratively electrically connected to substrate 303 via lead 304 having voltage source 305. An illustrative nanopost is shown in greater detail in FIG. 4. In that figure, nanopost 302 is electrically insulated from the liquid (301 in FIG. 3A) by material 401, such as an insulating layer of dielectric material. The nanopost is further separated from the liquid by a low surface energy material 402, such as a well-known fluoropolymer. Such a low surface energy material allows one to obtain an appropriate initial contact angle between the liquid and the surface of the nanopost. It will be obvious to one skilled in the art that, instead of using two separate layers of different material, a single layer of material that possesses sufficiently low surface energy and sufficiently high insulating properties could be used.
FIG. 3B shows that by, for example, applying a low voltage (e.g., 10-20 volts) to the conducting droplet of liquid 301, a voltage difference results between the liquid 301 and the nanoposts 302. The contact angle between the liquid and the surface of the nanopost decreases and, at a sufficiently low contact angle, the droplet 301 moves down in the y-direction along the surface of the nanoposts 302 and penetrates the nanostructure feature pattern until it complete surrounds each of the nanoposts 302 and comes into contact with the upper surface of substrate 303. In this configuration, the droplet covers surface area f2 of each nanopost. Since f2>>f1, the overall contact area between the droplet 301 and the nanoposts 302 is relatively high and, accordingly, the flow resistance experienced by the droplet 301 is greater than in the embodiment of FIG. 3A. Thus, as shown in FIG. 3B, the droplet 301 effectively becomes stationary relative to the nanostructure feature pattern in the absence of another force sufficient to dislodge the droplet 301 from the feature pattern. Such control is, in part, the subject of copending U.S. patent application Ser. No. 10/403,159, filed Mar. 31, 2003, entitled “Method And Apparatus For Variably Controlling The Movement Of A Liquid On A Nanostructured Surface,” and is hereby incorporated by reference herein in its entirety.
In yet another prior attempt, instead of nanoposts or microposts, a closed-cell nanostructured or microstructured surface is used in a way such that, when the pressure of a fluid within one or more of the nanocells or microcells of the surface is decreased, a liquid disposed on that surface is caused to penetrate the surface thus, for example, increasing the flow resistance experienced by the droplet. Such a closed-cell structure is advantageous in that, by increasing the pressure within one or more of the cells to or above a desired level, the liquid is forced back out of the cell(s) and is returned at least partially to its original, unpenetrated, low flow-resistance position. In this way, the penetration of the droplet into the surface can be varied to achieve a desired level of flow resistance experienced by the droplet of liquid. Such reversible penetration is, in part, the subject of copending U.S. patent application Ser. No. 10/674,448, filed Sep. 30, 2003, entitled “Reversible Transitions on Dynamically Tunable Nanostructured or Microstructured Surfaces,” and is hereby also incorporated by reference herein in its entirety.